1. Field of the Invention
The present invention relates to a technique for short-range data transfer between terminal devices used for compact and portable information terminal devices such as wireless handy terminals.
2. Description of the prior Art
Conventional short-range data transfer techniques for compact and portable information terminal devices utilize electromagnetic induction which occurs between electromagnetic coils in data transmission and reception portions. Such electromagnetic induction can be described as the relationship between an electrical field E and a magnetic field H given by the following equations (i.e. Maxwell's electromagnetic field equations). EQU rotE=-.mu.dH/dt (1) EQU rotH=-.epsilon.dB/dt (2)
In the above equations (1) and (2), A represents as a permeability; .epsilon. represents a dielectric constant; and t represents time.
FIG. 6 shows the scheme of the conventional short-range data transfer wherein electromagnetic coils are used in data transmission and reception portions. When an alternating current 26 including signal is applied to an electromagnetic coil 25 of a transmission portion 24, an a.c. magnetic field H is produced and an electrical field E is also induced according to Equation (1). In other words, when the alternating current 26 is applied to the electromagnetic coil 25 of the transmission portion 24, an electromagnetic wave 27 caused by interaction between the magnetic field H and induced electrical field E produced at the electromagnetic coil 25 of the transmission portion is radiated in a space. When the electromagnetic wave 27 radiated in the space is detected by an electromagnetic coil 29 of a reception portion 28, a current I 30 and a magnetic field are induced at the electromagnetic coil 29 of the reception portion 28 in accordance with Equations (1) and (2). The data at the transmitting end can be transferred to the receiving end by analyzing the induced current I 30. The conventional method of short-range data transfer techniques utilizing electromagnetic induction is summarized above. Such short-range data transfer techniques are widely used due to the simple circuitry employed, although the technique is less desirable in terms of transfer speed, transfer distance and the like than wireless communication techniques such as those used in portable telephones (mobile telephone systems) and radios.
Usually, it is required for a compact and portable wireless terminal to have the function of communication with the parent terminal and the function of communication with other offsprings. Such data transfer is carried out in a short-range such as several centimeters or in a state wherein the terminals are in contact with each other. In such situations, the electromagnetic induction system using electromagnetic coils as described above is required. Therefore, a compact and portable wireless terminal incorporates an analog circuit portion for transmission and reception of electrical waves in the range from several tens of MHz to several tens of GHz to be used for wireless communication (hereinafter referred to as RF circuit) and a relatively simple data transmission/reception circuit portion for performing short-range data transfer utilizing electromagnetic induction (hereinafter referred to as an electromagnetic induction circuit). As is apparent from Equations (1) and (2), a current I induced at the electromagnetic coil of the receiving end of the electromagnetic induction circuit is increased in proportion with the frequency of the signal to be transferred. Therefore, although the reception sensitivity is sufficient for transferred signals of high frequencies, the sensitivity is insufficient for low frequencies. This has resulted in a serious problem with data transfer systems using normal electromagnetic induction coils in that they can transfer only signals of high frequencies. Especially, transfer has been almost impossible for signals having frequencies of several tens of Hz or lower. This problem has necessitated designers to indiscriminately increase the frequency of signals to be transferred by short-range data transfer systems such as compact and portable wireless terminals in order to improve reception sensitivity. This has inevitably resulted in an increase in the power consumption of such an electromagnetic induction circuit which has created a serious problem in product design.